Static bipolar random access memories are well known for use in applications where important considerations are short access times for reading and writing, low vulnerability to alpha particle errors, and less volatility than is characteristic of dynamic random access memories.
The usual cell for use in static RAMs typically includes a bistable circuit including a pair of cross-coupled transistors and a pair of load transistors, and a pair of diodes or transistors for coupling the internal nodes of the bistable circuit to the input/output bit lines. Among the variety of cells of this kind proposed there is one which employs NPN transistors as the cross-coupled pair, PNP transistors as the load pair, and PNP transistors as the coupling pair. To decrease the cell size and keep the power dissipation low, there is extensive merging of the devices in a combined circuit layout design and there is used the principle of direct minority carrier injection both for the current supply and for coupling to the input/output lines. To this end the cross-coupled PNP transistors are designed as upside-down operated or inverse transistors with a common N plane acting as an emitter. It is usually difficult to achieve high gain in inverse transistors by the usual fabrication techniques, and in these prior art devices the low gain necessitates compromises in the design and makes for instability in the bistable circuit.
It has previously been recognized that Schottky diodes are useful for the coupling elements because of their low capacitance, negligible storage time and small size. However, their lack of gain is a deterrent to their use since gain is useful for increasing the stability of the bistable circuit. It has also been proposed that it would be preferable to drive the readout lines by a constant current source rather than a constant voltage source if it were convenient to do so.
Among the bipolar technologies which have been proposed for the effective implementation of merger or injection coupled logic is the so-called OXIL technology described in the IEEE Trans. on Electron Devices, Vol. ED-27, No. 8, August 1980, pp 1397-1401. This technology can provide vertical upside-down transistors for injection coupled logic which operate in the forward mode with consequent high gain.